Role of Staphylococcus aureus alpha-hemolysin in disease Staphylococcus aureus is the leading cause of bloodstream, lower respiratory tract, skin and soft tissue infections in the United States. Demonstrating the broad tissue range and virulence properties of the pathogen, S. aureus also causes osteomyelitis, septic arthritis, and a spectrum of toxin-mediated entities including staphylococcal toxic shock syndrome, enterotoxin-induced gastrointestinal disease, and life-threatening desquamation caused by a family of epidermolytic toxins. Recent estimates suggest that S. aureus contributes to half a million infections per year in the United States alone, resulting in nearly 20,000 deaths. The annual economic burden of S. aureus infection reached $14.5 billion in 2003, a rate of increase per annum of 11.9% when compared to 1998. The remarkable pathogenic potential of S. aureus has been demonstrated over the past 15 years by the rapid spread of highly virulent strains worldwide. Current epidemic strains harbor genes encoding for resistance to methicillin (MRSA), rendering the once highly potent class of 2-lactam antimicrobials obsolete as therapeutic agents. To date, there is no commercially available vaccine to prevent S. aureus infection, and novel antimicrobial agents that successfully target this organism have been few. In the context of widespread disease that has been met with a paucity of highly effective, durable anti-infective strategies, it is imperative that we obtain a more detailed understanding of the molecular mechanisms of S. aureus pathogenesis. S. aureus encodes an array of secreted toxins that contribute to host tissue injury. While many of these toxins are variably expressed in distinct strains, the pore-forming cytotoxin alpha-hemolysin (Hla) is encoded in the genome and expressed by almost all S. aureus strains. Hla is a potent epithelial toxin, contributing to the pathogenesis of pneumonia, skin and corneal infection, central nervous system infection, toxic shock syndrome and sepsis. The primary goal of this application is to develop a refined view of how Hla injures cells and epithelial tissues, leading to disease. This application is based on four fundamental discoveries: 1) Hla is required for the pathogenesis of pneumonia and other staphylococcal infections, and preventative and therapeutic strategies that antagonize toxin action afford protection against disease; 2) Hla binds to ADAM10 as its eukaryotic cellular receptor, thus, cell sensitivity to Hla is conferred by expression of ADAM10; 3) Hla utilizes the native cellular activity of ADAM10 to cause host tissue injury; and 4) ADAM10 is required for Hla- mediated lethal infection in the lung. Through studies that reveal the precise mechanism by which the Hla- ADAM10 complex results in host cell injury, we anticipate the discovery of novel, focused therapies that interfere with the fundamental elements of the host-pathogen interface. These studies are expected to shed light on elements of host susceptibility to S. aureus disease, and contribute more broadly to our understanding of bacterial pore forming cytotoxins.